Fibre optic distributed acoustic sensing (DAS) is a known technique where an optical fibre, deployed as a sensing fibre, is interrogated with interrogating radiation and radiation which emerges from the fibre is detected and analysed to determine environmental changes acting on the optical fibre. Some fibre optic sensors rely on deliberately introduced features within the fibre, e.g. fibre Bragg gratings or the like, to induce reflection from a point in the fibre. In a fibre optic distributed sensor however the radiation which is backscattered from inherent scattering sites within the fibre is detected. The sensing function is thus distributed throughout the fibre and the spatial resolution and arrangement of the various sensing portions depends on the characteristics of the interrogating radiation and the processing applied.
Various types of DAS sensor have been demonstrated including sensors based on Rayleigh scattering of light from the sensing fibre. Light transmitted into an optical fibre will be scattered from the various inherent scattering sites within an optical fibre. A mechanical vibration of the fibre, such as caused by an incident acoustic wave, will alter the distribution of scattering sites resulting in a detectable change in the properties of the Raleigh backscattered light. Analysing such changes allows relatively high frequency vibrations/acoustic stimuli acting on sensing portions of the optical fibre to be detected.
One type of DAS sensor performs repeated interrogations of the sensing fibre. Each interrogation involves transmitting at least one pulse of coherent optical radiation into the optical fibre and detecting the intensity of backscattered light from each of a number of sensing portions of the sensing fibre, also called channels of the DAS sensor. The intensity of backscatter from a given channel in response to separate interrogations of the sensing fibre is monitored to determine any acoustic stimulus acting on the fibre. In the absence of any environmental stimulus the backscatter intensity from any given sensing portion should remain the same for each repeated interrogation (provided the characteristics of the interrogating pulse(s) remains the same). However an environmental stimulus acting on the relevant sensing portion of the fibre will result in an optical path length change for that section of fibre, e.g. through stretching of the relevant section of fibre and/or a refractive index modulation. As the backscatter from the various scattering sites within the sensing portion of fibre will interfere to produce the resulting intensity, a change in optical path length will vary the degree of interference and thus result in a change in backscatter intensity. This change in intensity can be detected and used as an indication of a disturbances acting on the fibre, such as an incident acoustic wave.
Such DAS sensors, in which the measurement signal is based on intensity variations in the detected backscatter, have been advantageously employed in a wide range of applications. One issue with such sensors however is that relative intensity change in response to a given input stimulus will vary from channel to channel and can also vary for a given channel over time. In other words the gain of the channels is variable. This means that it can be difficult to determine quantitative information about the stimulus from such a sensor. Also typically such sensors typically do not provide any reliable detection of low frequency disturbances on the optical fibre.
One way of providing more quantitative information is to use interrogating radiation which consists of two pulses with each pulse being at a different frequency. This means that the backscatter received at the detector comprises backscatter from both pulses, which will interfere, and thus there will be a signal component at the frequency difference between the pulses. If the two pulses are spatially separated in the fibre then any environmental disturbance acting on the fibre between the pulses that leads to an optical path length change will result in a phase change in the signal at this frequency difference, which can be thought of as a signal at a carrier frequency. By an appropriate choice of carrier frequency this phase change can be detected and the amount of phase change can be related to the amplitude of the disturbance acting on the fibre. Such two-pulse phase-output DAS systems are very useful but as mentioned typically require two spatially separated pulses. As the spatial resolution of the sensor is related to the size and separation of the pulses in the fibre this means that short duration pulses are typically used. However short pulses mean that less overall light is injected into the fibre each interrogation with the result that less backscatter will be detected. To achieve the same spatial resolution a two pulse system would have to use shorter pulses than a one pulse system. This can reduce the effective range of the two-pulse based system compared to the one-pulse intensity-output system.